1. Technical Field
The present inventive concept relates to a data storage system, and more particularly, to a data storage system and a data retention method thereof.
2. Discussion of the Related Art
Semiconductor memory devices include volatile and non-volatile forms. Volatile semiconductor memory devices have fast read and write speeds, but lose stored content in the absence of power. In contrast, non-volatile semiconductor memory devices have relatively slow read and write speeds, yet they retain stored content when not powered. Thus, to store data in an environment where there may be no continuous supply of power, the non-volatile semiconductor memory device is used.
Nonvolatile semiconductor memory devices may include phase-change random access memory (PRAM), mask read-only memory (MROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), and electrically erasable programmable read-only memory (EEPROM). In general, MROM, PROM, and EPROM are programmed by the manufacturer, rather than by ordinary users. On the other hand, EEPROM is able to be electrically erased and programmed by an ordinary user, and thus it is widely used for system programming needing continuous updating or as an auxiliary memory device. Particularly, since a flash EEPROM (hereinafter, referred to as a flash memory device) has a higher degree of integration than a conventional EEPROM, it is very often used as a large-capacity auxiliary memory device.
A flash memory device periodically performs a retention operation so that it can retain data programmed to a plurality of its memory cells beyond a predetermined period of time.
In the retention operation, the flash memory device is connected to a host, internally measures a retention period and performs data retention in intervals of the retention period. However, if the flash memory device is in an offline state, e.g., not connected to the host, for a longer period of time than the retention period, the state of the data cannot be secured. Accordingly, there is a need to secure the state of data programmed to memory cells in flash memory devices in the offline state.